Sonic Methods to Detect Delamination in Concrete Bridge Decks

Sonic Methods to Detect Delamination in Concrete Bridge Decks Impact of Testing Configuration and Data Analysis Approach Hoda Azari, Deren Yuan, Soheil Nazarian, and Nenad Gucunski Several studies have shown that the USW and IE methods are effective to detect delamination, voids, and cracks within the decks (2–4). The effectiveness of these methods depends not only on how the data are collected and interpreted but also on practical considerations, such as the density of measurements and the way the data are presented and visualized. To test these practical considerations, extensive, combined IE-USW tests were carried out on a bridge deck constructed with various defects, including delamination. The obtained experimental results were then analyzed to evaluate the following:

Accurate assessment of the condition of concrete bridge decks leads to a bridge management system that is mechanistic and reliable, qualities of great importance to transportation agencies. In this study, two sonic–seismic methods—impact echo (IE) and ultrasonic surface waves (USW)—were used to characterize material properties and to identify fabricated delaminated areas inside a concrete bridge deck. The reliability of IE and USW is affected not only by the strengths and weaknesses of these methods but also by the ways in which data are collected, interpreted, and presented. The data obtained from the two methods were visualized in both traditional and checkerboard formats. The checkerboard format located the delaminated areas more precisely because the smoothing algorithm was avoided in this contouring method. This observation is more critical as the density of data collection is reduced. The evaluative power of each method and the power in combination, were also compared and discussed in terms of detectability and accuracy. The combined results from USW and IE were preferred to the individual results of each method because the combined results provided redundant and complementary data that reduced uncertainty in the identification of defects with no added overhead for field data collection. A relationship between the size of the defect and measurement density was recommended through evaluation of the bridge deck at three measurement densities. The measurement spacing should be equal to or less than the smallest defect deemed critical by the owner agency.

• Impact of data visualization and interpretation approaches on the outcome of these methods; • Detectability and accuracy of the IE, USW, and combined IE-USW methods; and • Impact of the measurement density on the detectability threshold of these methods. Description of Study Methods USW Method The USW method is used to estimate the average velocity of propagation of surface waves in a medium, such as a slab or bridge deck (Figure 1a). Modulus of the medium can then be calculated from the measured velocity on the basis of known or assumed density and Poisson’s ratio. In this method, the variation in velocity with wavelength is measured to generate a so-called dispersion curve. For a uniform or intact concrete deck, the dispersion curve shows more or less a constant velocity within the wavelengths that are less than the thickness of the deck. At a defective point (a point that contains delamination, a void, or deterioration), the USW average surface wave velocity (modulus) becomes less than the intact point because of interference from the defect. In this case, the modulus obtained may be called an apparent modulus. Figure 2a shows typical USW results for an intact area; a shallow, severely delaminated area; and an area of deep delamination. The dispersion curve shifted to lower values for the shallow and deep delamination.

Delamination induced by corrosion, stress, or both is one of the most common types of defects in concrete bridge decks. Several nondestructive testing techniques have been used to detect and characterize this type of defect. Sonic–seismic methods, including impact echo (IE) and ultrasonic surface wave (USW) are among these methods. Each technique has its own advantages and limitations in bridge deck condition assessment (1). Measurements can be performed with these two methods simultaneously with a number of devices, including a portable seismic pavement analyzer. This combination enhances the capability and reliability of the sonic methods to determine material properties and thicknesses, as well as to detect defects in bridge decks. H. Azari, D. Yuan, and S. Nazarian, Center for Transportation Infrastructure Systems, University of Texas at El Paso, 500 West University Avenue, El Paso, TX 79968. N. Gucunski, Center for Advanced Infrastructure and Transportation, Rutgers University, 623 Bowser Road, Piscataway, NJ 08854. Corresponding author: H. Azari, [email protected].

IE Method IE is one of the most commonly used nondestructive testing methods to detect defects in concrete decks (6). This method has its basis in an impact on a plate-like object, such as a bridge deck, with an impactor to generate stress waves at frequencies up to 20 to 30 kHz, and in the collection of signals by a receiver mounted on the surface

Transportation Research Record: Journal of the Transportation Research Board, No. 2292, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 113–124. DOI: 10.3141/2292-14 113

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FIGURE 1 Schematic illustration of test methods (5): (a) USW and (b) IE.

(contact IE). Several recent studies, such as that of Carino et al. (6), have shown that air-coupled IE, in which a small microphone is used to receive the IE signals, works as effectively as contact IE to locate delamination in a concrete deck (7). The state of the practice in the interpretation of the IE results consists of the transfer of the time–domain signal (waveform) to the frequency–domain (amplitude spectrum) with the use of a fast Fourier transform algorithm. The peak frequencies associated with the amplitude spectrum are used either to estimate the thickness of the deck, the depth of delamination, or the presence of a delaminated area. The deck thickness (h) is determined from a measured or assumed compression wave velocity (Vp) and the return frequency ( f ) from h=α

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where α is about 0.96 for concrete slabs. For a deep (deeper than 4 in.) and a less severe delaminated location, the return frequency may shift to a higher value that corresponds to the depth of the delamination. As shown in Figure 1b, a shallow or a deep but extensive delaminated area (i.e., dimensions of the severe delamination are greater than 2 to 3 times the depth) usually is manifested by a low-frequency peak, which indicates that a flexural mode dominates the frequency response. With this return frequency, Equation 1 falsely provides a deck thickness significantly thicker than is actually the case. Equation 1 is not applicable to measure the depth to the delaminated area when the flexural mode is dominant. Figure 2b represents the typical amplitude spectra

of (a) intact, (b) shallow delaminated, and (c) deep delaminated areas. Compared with the intact point, lower frequencies control the response at both the shallow and the deep, extensive delaminations. With appropriate hardware, proper filtering, and careful examination of the amplitude spectrum, the high-frequency peak related to the depth of a severely delaminated area can be detected in the amplitude spectrum, especially when the concrete surface is smooth and microcrack free. This process has been reported in academic circles but has not made its way in the state of practice.

Combined IE-USW Method IE in conjunction with the USW method can be implemented easily in the field with one of the integrated sonic–seismic devices, such as a portable seismic pavement analyzer. This analyzer consists of two receivers and a source packaged into a hand-portable device. The typical impact duration for this device is about 40 µs, and the data acquisition system has a sampling frequency of about 400 kHz per channel, which make the device quite suitable for depths of measurement greater than 3 in. The advantage of the combined methods in a single device is that, once a test is performed, the modulus and thickness of a concrete deck can be estimated concurrently. Besides, for shallow, or extensive, deep delamination, when the flexural mode of vibration dominates the IE tests, the depth of a defect can be approximately observed from the USW dispersion curve. The following sections discuss the acquired results from the two sonic methods, along with their data interpretation approaches.

Azari, Yuan, Nazarian, and Gucunski

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Average Modulus = 4192 ksi

2 Shallow Severe Delamination Deep Delamination

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(c) FIGURE 3 Views of fabricated bridge deck: (a) delamination installation, (b) concrete deck with supporting girders, and (c) test with portable seismic pavement analyzer.

Fabricated Bridge Deck and Measurements The concrete deck for this small-scale study was constructed in accordance with guidelines of the Texas Department of Transportation. The fabricated bridge deck was 20 ft long, 8 ft wide, and 8.5 in. thick and was supported by three 15-in.-wide concrete girders (Figure 3). The supporting girders were used to evaluate the concerns of the high-

way agencies with the effectiveness of some nondestructive testing methods on top of the girders. The 28-day compressive strength of the concrete was 4,000 psi. The deck was reinforced with two mats of Grade 60, No. 5 steel bars spaced at 8 in. centered in the transverse direction and 10 in. in the longitudinal direction. Nine artificial delaminated areas (Code DLs) of varying sizes, depths, and severity levels were built in the deck as presented in Table 1. Three synthetic fabrics were used to simulate different stages of delamination: from

TABLE 1 Detailed Information About Delaminated Areas in Fabricated Concrete Deck Defect Code DL1 DL2, DL3 DL4 DL5, DL6 DL7 DL8 DL9

Size (in.)

Depth (in.)

12 × 12 24 × 24 12 × 12 24 × 24 24 × 24 24 × 48 12 × 24

3 3 3 3 6.5 6.5 3

Note: DL = delamination.

Material and Remarks

Condition Type

Soft and high strength, thin (about 1 mm) foam

Progressed shallow delamination

Soft and high strength, thick (about 2 mm) foam

Severe shallow delamination

Soft and high strength, thin (about 1 mm) foam

Progressed deep delamination

Very thin (about 0.3 mm) and soft polyester fabric

Initial shallow delamination


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FIGURE 4 Plan view of distribution of delamination in fabricated concrete deck.

initial to severe. Figure 4 shows an overview of the approximate horizontal distribution of the defects as built in the deck. DL 8 and DL 9 together were considered a composite delamination. The solid dashed lines in the figure delineate the horizontal positions of the three supporting girders. The horizontal distribution of the defects was done on the basis of (a) convenience to set up uniform test points in the longitudinal and transverse directions, (b) minimization of interference between the defects, and (c) reserve of an 8-ft by 4-ft intact area for calibration purposes.

Data Representation and Analysis Detectability and Accuracy The deck was evaluated with use of a 6-in. grid. Because USW and IE are point inspection methods, it was more effective to visualize the results in a planar contour map than to evaluate them individually. The contour maps of the variations in the average moduli and return frequencies along the deck obtained by the USW and IE methods are presented in Figure 5. The indications of defects on intact areas typically corresponded to microcracks or edge effects. The smearing effects around the delaminated areas could be attributed to the contouring algorithm used. In Figure 5a, most defects, except for portions of the small or deep delaminated areas, manifested themselves as areas with lower average moduli. The variability of modulus through the entire deck might result from change of modulus after construction. Figure 5b presents the contour map of the peak frequencies of the IE amplitude spectra. A peak frequency of about 9.5 kHz corresponded to the frequency associated with an intact point. On the basis of an average compression wave velocity of 13,800 ft/s measured for the concrete, this peak corresponded to the deck thickness (8.5 in.) as shown in Equation 1. The flexural mode, with a peak frequency of about 3 kHz, was dominant for the shallow and deep delaminated areas. The location and size of the identified defects, except for the composite one, agreed well with the actual size. The two methods yielded similar defect maps for the large, shallow delaminated areas (both extensive and less severe ones). These complementary results built confidence in the interpretation of the defected

areas, especially in the conclusion that no additional time was needed to collect combined field data. On the basis of the average modulus contour plot, the USW method was less sensitive to the smaller and deeper defects. Besides the planar contour maps, which specified the horizontal boundaries, the line scans (B-scans) provided insight into the depth of the delamination (Figure 6). The USW B-scan is in the form of variation in modulus with wavelength, which can qualitatively be viewed as a scaled variation of modulus with depth. The USW B-scan along the longitudinal centerline is presented in Figure 6a. The locations and depths of large, shallow delaminated areas, but not the small ones, are approximated quite well. Also, the symptoms of deep delamination, in terms of lower modulus, are apparent, at the depth of 6.5 in., but they are not as obvious for the shallow ones. This finding indicates that the USW method was not effective to locate small and deep delaminated areas. The spectral B-scan of the IE results along the longitudinal center line is shown in Figure 6b. A black stripe at a frequency of about 9.5 kHz that corresponds to the bottom of the concrete deck (echo mode) is evident along the intact areas. The bottom reflection that corresponds to the right supporting girder is apparent, because the deck was not constructed as an integral part of the supporting girders. There was some loss of energy and weaker reflection from the bottom of the deck on the left supporting girder. Along the delaminated areas, the peak frequency was significantly lower than the fullthickness echo frequency of 9.5 kHz, which indicated that the flexural mode of vibration was dominant at those areas. In Figure 6b, some faint energy at about a frequency of 11 kHz was apparent at some points along the composite delaminated area, which corresponded to the 6.5-in. depth of the deeper defect. Comparison of the USW and IE contour maps indicated that both methods detected the shallow delaminated areas accurately and in a complementary manner. The IE method also showed considerable promise to locate the deep and small, shallow defects. The USW method could locate the existence and the approximate depths of shallow defects and the quality of concrete in terms of modulus. The USW method was more powerful than the IE method in the areas of supporting girders when the girders and the deck were in intimate contact. In such case, a weaker reflection from the bottom of the deck would be detected with the IE method.

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(b) FIGURE 6 Line scans from USW and IE along longitudinal centerline: (a) USW apparent modulus and (b) IE spectral B-scan.

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FIGURE 5 Contour maps of acquired results from USW and IE tests: (a) average apparent modulus from USW and (b) dominant frequency from IE.

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(b) FIGURE 7 Planar variation of apparent modulus at different depths: (a) 3 in. and (b) 6 in.

Finally, the planar contour maps of variations in modulus, obtained by the USW method at two depths, are shown in Figure 7. As indicated in Table 1, the shallow delaminated areas were located at a depth of about 3 in., and the deep ones were embedded at an approximate depth of 6.5 in. Indications of the decrease in apparent modulus were visible at a depth of 3 in. under the large, shallow delaminated areas; indication of lower moduli was minute or none for the small and shallow and deep delaminated areas (Figure 7a). The contour maps associated with a depth of 6 in. clearly contained the manifestation of lower moduli for the shallow, large delaminated areas; some indication was apparent of the existence of the deep and small, shallow delaminated areas (Figure 7b). These findings confirmed that the USW method was not effective to detect smaller and deeper delaminated areas. Impact of Contouring Algorithm A contouring algorithm is quite helpful to visualize the extent of defects but may distort some information contained in the raw data and introduce artifacts that may be misjudged as defects. Figure 8 shows three contouring algorithms used to represent the USW average moduli, namely, (a) traditional with unlimited color index, (b) traditional with two-color index, and (c) checkerboard. Traditional contouring uses a smoothing algorithm to ensure that the displayed contour lines change gradually and incrementally from a minimum value to a maximum value. A large number of shades

of black and white are used in the smoothing algorithm when the unlimited color index approach is selected. The two-color index contours contain only two colors delineated by a threshold value. A smoothing algorithm is used to depict the results. The checkerboard algorithm plots a rectangular array of cells. The value for each cell is determined by smoothing the results with the values of that cell and the four adjacent cells to define a surface rectangle. The mean (E ) of the measured moduli from all points, minus two times the standard error (σ) associated with the USW method, was used as the threshold value for the traditional with two-color index and checkerboard contour methods to plot USW results. In accordance with Nazarian et al., the standard error in the USW measurements for concrete slabs was 7% (8). The target modulus to delineate between the intact and delaminated areas [i.e., E (1 − 2σ)] was set at 0.86 E to ensure that the delaminated areas were selected with a level of confidence of about 95%. The test points with a modulus less than 0.86 E are shown in black to indicate that they were defective. The typical contouring algorithm with unlimited color index is shown in Figure 8a. Figure 8b, which corresponds to the typical contouring algorithm with a two-color index, gives a clearer picture of the small, shallow defects and the deep ones. Some smearing effect in the results remained, whereas representation of the data in a checkerboard format seemed to enhance the evaluative power of the results (Figure 8c). In other words, the checkerboard contouring format reduced the false positives noticeably, because the smearing effect was avoided in this type of contouring. Similarly, the IE dominant frequency



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FIGURE 8 Contouring algorithms to present average apparent modulus by USW: (a) traditional contour map with unlimited color index, (b) traditional contour map with two-color index, and (c) checkerboard contour map.

contour maps are presented in Figure 9 with the same three contouring formats. Because the smearing effects were small (Figure 9a), the accuracy of the results among the outcomes of the different contouring algorithms was similar. The IE method showed promise to detect the deep delamination and the composite one. However, both methods had similar power to identify the shallow defects. The USW method provided some approximate information about the depth of the defect.

In addition to the subjective assessment of the contour maps, the accuracy of the methods was judged objectively through correlation of the obtained responses to the conditions of the point tested (intact versus delaminated). For this purpose, each cell was graded 0 or 1. A grade of 1 was assigned to each intact point that was detected as intact, or a delaminated point that was detected as delaminated. If the reported result did not correspond to the actual condition of the deck

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FIGURE 9 Contouring algorithms to present dominant frequency by IE: (a) traditional contour map with unlimited color index, (b) traditional contour map with two-color index, and (c) checkerboard contour map.

at a given point (false positive or false negative), the point was graded 0. On the basis of these criteria, the predictions of the USW and IE methods were correct for about 83% and 85% of the points tested, respectively. Similar procedures and calculations were then applied to the results of the two methods simultaneously. On the basis of the criteria described, the detectability of the combined results to locate

the defects enhanced slightly to 86%. However, additional confidence in the reported results, specifically for small, shallow delaminated and deep delaminated areas, may have outweighed the slight overhead required for the joint analysis of the data. On the basis of this study, the joint analysis was more robust for older bridge decks whose slabs contained cracks and degraded concrete.

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Effect of Measurement Density

measurement spacing corresponded to testing frequencies equal to, and twice, the smallest defect, respectively. As reflected in Figures 10 and 11, when the frequency of the measurement decreases, the resolving power of the USW and IE methods diminishes, and the probability increases that smaller defects will be missed. Most of the defects were detected by the USW and IE methods for the 6-in. measurement spacing shown in Figures 10a and 11a. As reflected in Figures 10c and 11c, the coarse measurements with 2-ft spacing missed most defects, because few measurements have been made on top of defects.

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One major concern in the evaluation of bridge decks is how dense the measurements should be to obtain optimal results. The USW average modulus and the IE dominant frequency contour maps for three measurement densities are shown in Figures 10 and 11, respectively. Because the smallest dimension of the embedded delamination was 12 in., the measurement spacing of 6 in. corresponded to testing frequency equal to one-half the smallest defect. Similarly, the 1-ft and 2-ft

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(c) FIGURE 10 Planar variation of average apparent modulus at different measurement densities: (a) every 6 in., (b) every 1 ft, and (c) every 2 ft.

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On the basis of these results, it was assumed that the measurement spacing should be equal to or less than the smallest delaminated area to be detected by either the USW or IE method. To map the delaminated area accurately, the measurement spacing should be half the desired smallest dimension of the area that is of practical value. These results supported the value of an initial, coarser measurement on an actual bridge deck, to be followed by more thorough secondary measurements.

Conclusions The USW and IE methods proved effective to locate and characterize the defective areas in a bridge deck, specifically when the results were evaluated in combination. The USW and IE methods identified the shallow delaminated areas. In addition, the IE method showed promise for detection of the small, shallow delaminated areas. The depth of shallow defects could be approximated from

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the USW dispersion curve B-scans. Therefore, the combined IEUSW method could detect the horizontal and vertical locations of delaminated areas quite well. The USW and IE methods exhibited accuracy of detection of better than 80% of the delaminated areas. The combined method did not seem to improve the level of accuracy, but it did improve confidence in the detectability of the delaminated areas as a result of the complementary nature of the two methods. The USW and IE B-scans provided more reliable information about the embedded delamination than the planar surface contours. The evaluative power of both methods depended not only on how the data were collected and analyzed but also on how the data were presented. Traditional contouring was quite successful to locate defects. As a result of smearing effects, however, traditional contouring might interpret some intact areas as defective, especially in the vicinity of the defects. Presentation of the outcomes in a checker board contouring format, in which the smoothing was minimized, could improve confidence in data interpretation. The dimensions of the smallest size defect that could be estimated seemed to be equal to the measurement interval. To optimize investigation funds, the measurement interval should be established on the basis of the smallest practical defects that would substantially affect the decision about the maintenance or rehabilitation of a bridge deck. Acknowledgment This study was conducted on a deck constructed as part of SHRP 2 Project R06(A), Nondestructive Testing to Identify Concrete Bridge Deck Deterioration.


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